I use ProFTPD-1.3.1 with mod_clamav 0.5 and I have HiddenStores turned on
(comment on this directive?)

It seems that clamd ocasionally does not return the result in time and
server then fails to rename the file and also to remove it. The result is
that there's non-empty file named .in.<file>. left in destination directory
which makes following uploads impossible (unless the user finds and deletes
the temporary file. The log says: